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2-stage bleaching of oxygen delignified HW kraft pulp to full brightness : Practically possible by changing D0 to D

Pulp & Paper Canada, 2005-04, Vol.106 (4), p.56-60

2005 INIST-CNRS ;Copyright Southam Business Communications, Inc. Apr 2005 ;ISSN: 0316-4004 ;EISSN: 1923-3515 ;CODEN: PPCADD

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  • Title:
    2-stage bleaching of oxygen delignified HW kraft pulp to full brightness : Practically possible by changing D0 to D
  • Author: RAGNAR, M
  • Subjects: Applied sciences ; Chlorine compounds ; Exact sciences and technology ; Lignin ; Oxygen ; Paper, paperboard, non wovens ; Polymer industry, paints, wood ; Pulp & paper mills ; Pulp bleaching ; Pulp manufacturing ; Technology ; Wood. Paper. Non wovens
  • Is Part Of: Pulp & Paper Canada, 2005-04, Vol.106 (4), p.56-60
  • Description: A mill-oxygen-delignilled Eucalypt kraft pulp (the raw material constituting of about 70% E. Grandis and 30% E. Saligna) with kappa 9.9 and viscosity 940 dm^sup 3^/kg was used as the raw material in this study. A wash loss of 6 kg COD/BDMT (kilogram COD per bone dry metric ton of pulp) was present in the pulp entering the first stage. This pulp was bleached in a number of two-stage and three-stage sequences: D(EO)D, D(EOP)D, (DQ)(PO), (DQ)(PO)D, (AQ)*(PO)D, (DQ)*(PO), D*(EO)D, and (AD)*(EO)D at 10% pulp consistency in all the stages. Chlorine dioxide and hot acid treatments were performed in sealed plastic bags heated in a thermostated water bath after intense kneading. Pressurized peroxide stages in addition to (EO)-stages were performed in Teflon-lined stainless steel autoclaves pressurized to 0.5 MPa with oxygen and heated while rotating in a polyethylene glycol bath. The asterisk "*" denotes treatment with high temperature (90°C) and long retention time (120 min)1, for D* according to the DUALD concept by Kvaerner Pulping (a further development of ideas originally presented by Lachenal and Chirat [12]). The A* stage relates to the YELLOW concept by Kvaerner Pulping or the AHL-stage according to Andritz (the technology described in a patent by Mo & Domsjo AB from the early 1960s [13]. The ISO brightness target brightness of the bleaching was 89.0%. For each sequence, the chemical charge in the last stage was varied to obtain a curve with three points, to allow a graphic evaluation by interpolation of the bleaching chemical requirement. Kappa number (C 1:00), intrinsic viscosity (CM 15:99) and AOX (W 9:89) were determined according to SCAN test standard methods, and ISO brightness was determined according to ISO 2470:1999. Brightness reversion'was evaluated by measuring the loss in ISO brightness units after free drying in an oven at 1050C for 3 h. After the heat treatment, the pulp was cooled to room temperature in a conditioning room with a controlled humidity, and the brightness was measured after 3 h in this atmosphere. The method is in close agreement with TAPPI's "useful method 200," the only difference being that TAPPI recommends four hours of heat treatment. It was decided to report the yellowing in terms of loss of ISO brightness units and not as PC number for two reasons: 1) because the reference level was the specific ISO brightness level of 89.0% for all the pulps compared, and 2) because ISO brightness units are of interest from a practical perspective. The pressurization and high temperature of the treatment achieved a very efficient brightness increase to in a limited time scale of some 1-2 hours. At first the pressurized peroxide stages were placed in sequences with several chlorinedioxide stages or in pure TCF sequences (TCF=Totally Chlorine Free bleaching). Since TCF is currently gaining little interest, it is of great importance primarily to look at ECF applications of pressurized peroxide bleaching (ECF=Elemental Chlorine Free bleaching). In the mid 1990s, two Swedish mills turned to operating a three-stage bleaching sequence of DQ(PO) for their production of fluff and top-layer for liquid board, respectively. Both mills run on softwood and the goal of the bleaching is not full brightness, but rather to reach an ISO brightness in the range 85-88%. To reach full brightness for softwood kraft pulp with only two bleaching stages, as in these cases, proves quite tough. However, for hardwood kraft pulp the situation is different. In general, HW kraft pulp is brighter than SW pulp at a given kappa number. This is at least partly an effect of different amounts of 4-deoxy-(-L-threo-4-enopyranosyluronic acid, generally referred to as hexenuronic acid (HexA) in the different pulps, since the HexA structure is more or less uncoloured in the visible region [3]. The three-stage sequence DQ(PO) should, thus, be more promising for application to HW bleaching, and even more economical would be the twostage sequence of (DQ)(PO), where the chelation is not carried out in a separate vessel and where only two washers are sufficient for the whole bleach plant. Combining D and Q proves unproblematic, since the chelatant is charged in the standpipe after the reactor where the entire chlorine dioxide has reacted. The idea as a whole was thus bright, but reality unfortunately proved more complex.
  • Publisher: Don Mills, ON: Southam
  • Language: English
  • Identifier: ISSN: 0316-4004
    EISSN: 1923-3515
    CODEN: PPCADD
  • Source: ProQuest Central
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